This invention relates generally to the field of directional elements for use in fault direction determinations for electric power systems, and more specifically concerns such a directional element useful for ungrounded power systems.
In an ungrounded power system, there is no intentional ground. Such a power system will typically include a number of distribution lines, each of which services a plurality of feeder loads, which are connected phase-to-phase. When a ground fault (a fault involving one phase of the power signal and ground) occurs on such a system, the only path for the ground fault current is through the distributed line-to-ground capacitance of the surrounding portion (relative to the fault location) of the power system as well as the distributed line-to-ground capacitance of the two remaining unfaulted phases of the faulted circuit.
It is well known that ground faults which occur in ungrounded power systems do not affect the phase-to-phase voltages between the three power signal phases (VA, VB, VC), so that it is possible to continue operating the power system while it is in the faulted condition. In order to continue operating, however, the system must have suitable phase-to-phase insulation and all loads on the system must be connected phase-to-phase.
The fault current for ground faults in ungrounded systems is quite low when compared with grounded systems, and hence, protective relays used for determining ground faults require high sensitivity. Most ground fault detectors (elements) used for ungrounded systems use fundamental frequency voltage and current components in their fault determinations. The conventional wattmetric method is a common directional element solution, but its sensitivity is limited to relatively low fault resistances, i.e. typically no higher than a few kilohms. Other methods use the steady-state harmonic content of current and voltage values, while still other methods detect the fault-generated transient components of both voltage and current to make ground fault determinations. These methods, however, have limited sensitivity because high resistance faults reduce the level of the steady-state harmonics and dampen the transient components of both voltage and current.
Ungrounded power systems do have many of the desirable characteristics of grounded systems, including safety, reduction in communication system interference and decrease in equipment voltage and thermal stress. One of the desirable features of ungrounded systems, as indicated above, is its relatively low ground fault current, so that the system can remain operational during sustained low magnitude faults, without presenting a safety risk to the public.
Ground faults in ungrounded systems often self-extinguish. However, there are some faults which do not self-extinguish; it is desirable to ascertain the existence and location of such faults as well as their direction so as to prevent the possibility of another, later fault combining in some way with the first fault to produce an extremely large fault current.
One difficulty with fault determination in ungrounded systems is the identification of the feeder which is faulted when that feeder is part of a multiple feeder distribution network. In balanced :systems (where the feeder lines are approximately the same length) the magnitude of zero sequence current provides a reliable identification of the particular feeder location of the fault. This is because in balanced systems, the capacitance along each feeder line is approximately the same. In unbalanced systems, such a magnitude value is not per se a reliable indication of which feeder contains the fault.
It would hence be desirable to have a reliable directional element for determining the existence of a fault in a multiple feeder distribution network, as well as the direction of that fault, in an ungrounded system, where the fault current is quite small.
Accordingly, the present invention is a directional element for detecting ground faults on ungrounded systems, comprising: means, when enabled, for calculating a zero sequence impedance for a particular protected line, using zero sequence voltage and zero sequence current on said line; an enabling circuit permitting operation of the calculation means under preselected current conditions; and means comparing the zero sequence impedance values from the calculation means with selected sensitive threshold values appropriate for ungrounded systems, wherein a forward fault indication is provided when the zero sequence impedance is above a first sensitive threshold and a reverse fault indication is provided when the zero sequence impedance is below a second sensitive threshold.